Fractional distillation of crude oil gives fractions in fixed amounts, but these do not match what people need. There is more demand for small molecules such as petrol than the crude oil supplies. There is a surplus of large molecules such as fuel oil and bitumen. Large molecules have high boiling points, are thick and viscous, and are difficult to ignite, so they are less useful as fuels.
In the fractional distillation practical, synthetic crude oil is heated and the fractions are collected at different temperature ranges. The fraction with the shortest molecules has the lowest boiling point, is the least viscous and is the easiest to ignite. The fraction with the longest molecules is dark, runs slowly and is hard to light.
Cracking solves the problem by turning surplus large molecules into smaller alkanes for petrol. It also makes alkenes, which are used to make polymers such as poly(ethene) and other chemicals. So cracking matches supply to demand and gives raw materials for the chemical industry.